This invention is generally directed to wind power utilizing systems and methods, using airborne wings or sails.
Recently, a novel approach to wind power utilization has appeared. A computer controlled kite, flying crosswind, harnesses power of the wind, which is further converted into electric energy or into propulsion of a ship. One example of former is U.S. Pat. No. 8,080,889 by Ippolito et al (assigned to KiteGen). One example of later is U.S. Pat. No. 7,672,761 by Wrage (assigned to SkySails). The common part is that the kite moves cross wind with high speed in so-called ‘figure 8’ trajectory. The tether of the kite also moves crosswind and experiences very large drag, which can exceed the drag of the kite itself. This drag wastes energy and limits possible length of the tether.
The crosswind flying airborne wing develops high lift forces. In the electricity generating applications, the speed of the tether, transferring this lift to the rotor of the electric generator, is relatively low (typically about ⅓ of the wind speed), resulting in relatively low power output for the force. This issue is further exacerbated by unwinding the tether from a tether drum, and using the same drum as a rotational element, converting linear motion of the cable into rotational motion. The drum is wide, and its width further increases when the tether's thickness increases. Consequently, drum's RPM is low and it requires an expensive gearbox with high input torque and large conversion ratio in order to achieve 1,500-1,800 RPM, required by most conventional electric generators.
One attempt to solve the problem of high cable drag is U.S. patent application Ser. No. 12/154,685 Faired Tether for Wind Power Generation Systems by Griffith et al. Unfortunately, the tether in that application is prohibitively expensive or inefficient.
This invention is directed to solving these problems and more.